Material system for tailorable white light emission and method for making thereof

ABSTRACT

A method of processing a composite material to tailor white light emission of the resulting composite during excitation. The composite material is irradiated with a predetermined power and for a predetermined time period to reduce the size of a plurality of nanocrystals and the number of a plurality of traps in the composite material. By this irradiation process, blue light contribution from the nanocrystals to the white light emission is intensified and red and green light contributions from the traps are decreased.

RELATED APPLICATION

[0001] This application is a division of U.S. application Ser. No.09/664,942, filed Sep. 19, 2000, entitled “Material System ForTailorable White Light Emission And Method For Making Thereof.”

[0002] The United States Government has rights in this inventionpursuant to Contract No. W-7405-ENG-48 between the United StatesDepartment of Energy and the University of California for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

[0003] The present invention relates generally to materials suitable foremitting electromagnetic radiation when suitably excited, and morespecifically, further relates to tailoring such emissions by thematerials.

BACKGROUND OF THE INVENTION

[0004] In the flat panel display field, liquid crystal displays (LCDs)are one of the preeminent display technologies and will continue to playa major role in flat panel displays. An important component of LCDs isthe white light emitter that comprises the back light for the displaysince liquid crystals (LCs) do not generate light—they may only blockit. Typically, LCDs allow 5-25% of the back light to pass through. As aresult, LCD technology requires a significant amount of energy, and thisis an important consideration in lightweight laptop or other displaydesigns. An efficient and spectrally broad white light source wouldconstitute an important contribution to LCD technology.

SUMMARY OF THE INVENTION

[0005] Aspects of the invention include a method comprising: directingan energy beam at a pre-processed composite material having a matrixcontaining a plurality of nanocrystals and a plurality of traps toreduce the size of said plurality of nanocrystals and the number of theplurality of traps to produce a post-processed composite material.

[0006] Aspects of the invention further include a method of tailoringwhite light emission from a composite having optical properties usingzinc selenide (ZnSe) nanocrystals comprising: fabricating the ZnSenanocrystals; incorporating the ZnSe nanocrystals into the matrix; andtuning the optical properties of the composite to a predeterminedapplication.

[0007] Aspects of the invention further include a material systemcomprising: a plurality of nanocrystals; a plurality of first and secondtraps; and said plurality of nanocrystals, first traps and second trapscapable of emitting white light in combination when excited.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The accompanying drawings, which are incorporated into and form apart of the disclosure,

[0009]FIG. 1A shows a composite material before the processing stepsdisclosed herein;

[0010]FIG. 1B illustrates a light emission spectrum of the pre-processedcomposite material of FIG. 1A on a linear scale;

[0011]FIG. 2 shows the composite material of FIG. 1A being irradiated;and

[0012]FIG. 3A shows a post-processed composite material after theirradiation process;

[0013]FIG. 3B illustrates a light emission spectrum of thepost-processed composite material of FIG. 3A on a linear scale;

[0014]FIG. 4 illustrates a flow diagram of an iterative processingmethod of the present disclosure;

[0015]FIG. 5 discloses a white light source for general lightingapplications using the post-process composite material;

[0016]FIG. 6 illustrates a liquid crystal display (LCD) using thepost-process composite material as the white emitting back light;

[0017]FIG. 7 illustrates a light emitting diode (LED) using thepost-process composite material having a various colors; and

[0018]FIG. 8 illustrates a pixel of a full color electroluminescentdisplay composed of a plurality of sub-pixels each including apost-processed composite.

DETAILED DESCRIPTION

[0019] Typically white light emitting devices do so throughcontributions of several spectral components, usually red, green andblue color light. Nanocrystals embedded in a matrix to form a compositematerial may be used in these light emitting devices. Nanocrystals aredefined as single crystal particles having average dimensionsapproximately in the range of 1 to 20 nanometers (nm), and typicallyapproximately 2 to 6 nm, but whose dimensions are ultimately determinedby the nanocrystal material and dimensions required to effect quantumconfinement. Quantum confinement is the shifting of energy levels tohigher energies as the particle size decreases. Due to their small size,nanocrystals confine carriers (electrons and holes) three-dimensionallyso that the effect of the quantum confinement of carriers may beobtained. The use of the terms “hole” and “holes” herein is intended torefer to vacant electron energy states, typically near the top of anenergy band, in a solid. Quantum confinement causes the energy of thelight emitted to increase as the size of the nanocrystal decreases, orequivalently, quantum confinement causes the wavelength of the lightemitted to decrease as the size of the nanocrystal decreases. The exactsize of the nanocrystals is dictated by the color of light to begenerated. Blue light, for example, requires smaller nanocrystals thanred light. A composite containing the nanocrystals may then be energizedby several different types of energy sources (e.g., a light source,electrical current, or electron beam) so that fluorescence may beinduced.

[0020]FIG. 1A illustrates a pre-processed composite material 10containing nanocrystals 14 in a matrix material 12 in a first statebefore the processing steps described herein. The height, h, of thepre-processed composite material may range from approximately 1 to 10millimeters (mm), the width, w, of the pre-processed composite materialmay range from approximately 1 to 10 mm, and the depth, d, may alsorange from approximately 1 to 10 mm. The matrix material 12 may be apotassium borosilicate glass matrix. However, other matrix materials mayalso be used that allow the incorporation of nanocrystals and which giverise to impurities or traps 16 and 18. Reference numerals 16 and 18 areused herein to indicate that there are at least two types of traps inthe matrix material 12 that emit in the visible light range as will bedescribed below. Reference numeral 16 indicates “red” traps that emitlight in the red region and reference numeral 18 indicates “green” trapsthat emit light in the green region. Traps 16 and 18, as used herein,are defined as any species such as impurities or defects that are notthe nanocrystals (though they may be contained within the nanocrystals)and which may either be excited by an energy source to produce lightemission or may trap excited carriers (electrons or holes) from thenanocrystals to produce light emission.

[0021] The pre-processed composite material 10 of FIG. 1A is constructedin the following manner. ZnSe nanocrystals 14 in a potassiumborosilicate glass matrix 12 are prepared by first melting a base glasscomposition formulated specifically for compatibility with Group II-VIsemiconductors. The base glass consists of (in weight (wt) %): 56%silica, 24% potassium oxide, 9% barium oxide, 8% boron oxide, and 3%calcium oxide. Twenty-five to thirty gram batches of this oxide powdermixture are melted in alumina crucibles and refined to remove bubbles at1400° C. for several hours. Following melt casting, the glasses areground into a fine powder. Next, ZnSe powder is added and the blendedmixture is re-melted again at 1400° C. for approximately one and a halfhours before casting the melt into small slabs. Excess ZnSe is added tocompensate for the expected volatilization losses from the meltingprocess. As-cast samples may appear reddish orange after overnightannealing at approximately 350° C. High-resolution transmission electronmicroscopy (HRTEM) on these samples should show crystalline ZnSeparticles with varying nanometer sizes. Typical sizes obtained are5.5±1.7 nm. These as-cast samples may then be successively cycledthrough re-melting and rapid quenching. HRTEM should show an averagediameter particle size for quenched crystalline ZnSe particles of3.7±1.1 nm. As discussed above, in addition to the ZnSe nanocrystals 14in the matrix material 12, the pre-processed composite 10 also containsat least two types of traps 16 and 18.

[0022] The method and device of the disclosed embodiments are concernedwith controlling the properties of the nanocrystals and the traps of thepre-processed composite 10 to tailor the blue, red and green contents ofthe light emission of the pre-processed composite material 10 to controlthe white light emission of post-processed composite material (referencenumeral 30 in FIG. 3A). The white light emission of the post-processedcomposite material 30 will be tailored (or tuned) by controlling each ofthe contributions of the blue, red and green components a predeterminedlevel. Depending on the specific application (e.g., indoor lighting,LCD, etc.) for which the post-processed composite 30 is to be used, thecontributions of the blue, red and green components of the white lightwill be predetermined.

[0023] The blue spectral content of the light emission of thepre-processed composite 10 is provided by the nanocrystals and may becontrolled by controlling the size of the nanocrystals. This is due tothe fact that the blue-shift in the emission and absorption spectra ofthe nanocrystals increases as the particle size decreases due to quantumconfinement. In the pre-processed composite material 10, the ZnSenanocrystals 14 in the matrix material 12 are in a size range wherequantum confinement of carriers (electrons and holes) may occur. Quantumconfinement in this range will shift the energy levels of the ZnSeconduction band and valence band apart and hence give rise to a bluelight contribution from the nanocrystals to the light emission. The bluespectral content of the light emission may also be controlled bycontrolling the number (or density) of the nanocrystals in addition tocontrolling the size of the nanocrystals.

[0024] The red and green spectral contents of the light emission of thepre-processed composite 10 may be controlled by controlling the number(or density) of red and green traps 16, 18. The traps 16, 18 are capableof trapping carriers and will decrease the efficiency and intensity ofthe blue light emission if the number of these traps are not reducedduring processing to improve the blue light emitting efficiency andintensity of the pre-processed composite 10. These traps may takeseveral forms that, in the case of ZnSe nanocrystals, may includecertain selenium (Se) molecules (e.g., Se₂ ⁻), selenium (Se) vacanciesand zinc (Zn) vacancies. By reducing the number of these red and greentraps 16 and 18, the amount of red and green spectral content of thelight emission from the post-processed composite 30 may be reduced andthe emission of the blue spectral content will be increased.

[0025]FIG. 2 discloses the pre-processed composite 10 being irradiatedand FIG. 3A shows the effects after irradiation on the post-processedcomposite 30. The exact makeup of the pre-processed composite 10 withrespect to nanocrystals and traps may vary before the processingdisclosed herein. However, a consistent white light emission is desiredfrom the post-processed composite 30 depending on a specificapplication. Indoor lighting, liquid crystal displays, and lightemitting diodes are just some of the specific applications that thepost-processed composite 30 may be used in and the white light emissionof these specific applications will also vary.

[0026] Therefore, a first step in the method disclosed herein is toobtain a spectrum of the emitted light for the pre-processed composite10 using an optical excitation source (e.g., laser, incandescent light).The spectrum of the emitted light is obtained nondestructively andwithout changing the properties of the pre-processed composite 10. Anexample of the spectrum of the emitted light of a pre-processedcomposite 10 is in FIG. 1B. Reference numeral 40 indicates weak bluelight emission from the pre-processed composite 10 due to the presenceof a large number and density of traps 16, 18 which trap the carriersemitted from the nanocrystals 14 and reduce the blue emission. Referencenumeral 42 indicates the spectrum of the emitted light from the largenumber and density of traps 16, 18 in the pre-processed composite 10. Asmay be observed in FIG. 1B, emission in the red and green spectral rangefrom the red and green traps 16, 18 dominates the overall light emissionfrom the pre-processed composite due to the large number and density ofthe traps 16, 18.

[0027] In a second step, an analysis of the results of the spectralevaluation process discloses the amount of irradiation the preprocessedcomposite 10 will require to produce a white light emission for aspecific application.

[0028] In a third step, the pre-processed composite 10 is irradiated fora predetermined amount of time. The irradiation may be performed usingan optical energy source. Examples of suitable energy sources includelasers, incandescent lamps, arc lamps, electron beams, and other typesof optical power sources. As discussed above, the power applied and theduration of the irradiation step will be predetermined based on thespecific application or end-use that the pre-processed composite 10 isbeing designed for and the results of the initial spectral evaluationstep. The power used during the irradiation may range from approximately10 milliWatts (mW) to 10 Watts. The duration of the irradiation mayrange from approximately 1 to 30 minutes.

[0029]FIG. 2 illustrates, for exemplary purposes, an argon ion laser 20irradiating on all lines 22 the pre-processed composite 10. In theexample shown, to change from a pre-processed composite 10 to a whitelight emitting post-processed composite 30 as shown in FIG. 3A requiresirradiation having power of approximately 53 mW of power forapproximately 2 minutes using all lines of an argon ion laser.

[0030] There are at least two factors or variables which determine theoptimum power and time duration of the irradiation step to achieve thedesired spectral makeup of the white light emission of thepost-processed composite 30. First, the decrease in the size of the ZnSenanocrystals and, second, the decrease in the number (or density) of redand green traps 16, 18. As shown in FIG. 3A, the greater power used andthe longer the pre-processed composite 10 is irradiated, the sizes ofthe ZnSe nanocrystals are decreased and, as a result, blue emission ofthe white light emission is increased. This result may be explained bythe fact that the oscillator strength, which affects the intensity andefficiency of light emission, of quantum confined nanocrystals generallywill have increased as the nanocrystal size decreases. Smallernanocrystal sizes are required to generate blue light. Therefore, as aresult of the quantum confined nature of the blue light emission, a highefficiency for the blue component may be expected in the post-processedcomposite 30. Also, as shown by FIG. 3A, the greater power used and thelonger the pre-processed composite 10 is irradiated, the less red andgreen trap emission will result from the red and green traps 16, 18because their number (or density) is reduced. The decrease in red andgreen trap emission not only produces less red and green spectralcontent to the white light emission from the post-processed composite30, but also more blue emission is produced which previously is beingtrapped by the large number of red and green traps 16, 18. The red andgreen light may result from trapping of the excitation energy initiallyabsorbed by the ZnSe nanocrystals that do not emit blue light.Therefore, most of the excitation energy given to the ZnSe nanocrystalswill either be emitted as blue, green or red, and therefore, the overallefficiency of white light emission should be high. Defining theefficiency as the number of blue, green or red photons emitted by thecomposite divided by the number of excitation photons absorbed by thecomposite, the efficiency of the post-process composite 30 will beapproximately in the range of 50 to 90% and, typically, greater thanapproximately 80%. The reduction in the red and green spectralcomposition of the white light emission and the increase in the bluecomponent of the white light emission are shown by reference numeral 44in FIG. 3B.

[0031] After the irradiation step, another spectrum of the emission fromthe pre-processed composite is taken. If the spectral content does notmeet the requirements of the specific application, multiple iterationsmay achieve the desired result. FIG. 4 illustrates the iterative stepsin flow diagram from. In step 410, the spectrum is taken. In step 412, adetermination is made whether the spectrum is appropriate for thespecific application. If the determination is positive, an efficientwhite light emitter composite has been created. If the determination isnegative, in step 414, the pre-processed composite 10 is furtherirradiated. The number of iterations may typically range fromapproximately one to ten iterations depending on the specificapplication. However, it is to be understood that the number ofiterations is not to be limited to ten. As previously discussed, thenumber of iterations will depend on the nature of the pre-processedcomposite and the specific application for which it is to be used.

[0032] In alternative embodiments, the blue light contribution to thewhite light emission may also be controlled by decreasing the number (ordensity) of the nanocrystals 14 in the matrix 12 in addition todecreasing their size.

[0033] In alternative embodiments, the nanocrystals 14 may also beselected from a group of similarly wide bandgap materials such as fromGroup II-VI, Group III-V, and Group IV semiconductor materials capableof emitting visible light upon excitation. Other examples of suitablenanocrystals may include CdSe and CdS.

[0034] In alternative embodiments the material used for the matrix 12may be from a group that is of a transparently visible material suitablefor having nanocrystals embedded within. Specifically, materialsincluded in this group may include polymers (e.g., polystyrene), gelsthat have been solidified in a particular manner (e.g., sol-gels such assilica sol-gel), and materials having traps that emit light in the redand green spectral ranges.

[0035] In alternative embodiments the traps may be from a group ofmaterials that emit visible light that may compliment the blue emissionfrom the nanocrystals to give white light emission. Such trap materialsmay emit cyan and yellow light.

[0036] The resulting processed composite material may be used in a widevariety of applications including full color flat panel displays,scanners, facsimile machines, copy machines, optical data storagedevices, internal lighting applications, light-emitting diodes (LEDs),automobile interior and exterior lighting, traffic safety lights, toys,and other general lighting purposes. In all these cases, an efficientand robust white light emitter is essential for high brightness, lowweight and long operational life.

[0037]FIG. 5 discloses a spectrally broad white light source for generallighting applications such as internal lighting for homes, automobiles,toys and exterior lighting. The power supply 52 provides the energysource to excite the post-processed composite 30 to give a white lightemission. The power supply 52 may be from a stationary power supplysource such as a conventional electrical socket or a mobile power supplysuch as a battery or batteries.

[0038]FIG. 6 illustrates a liquid crystal display (LCD) 60 using thepost-process composite material 30 as the white emitting back light. Thepower supply 52 provides an energy source to excite the post-processedcomposite material 30 in the LCD 60. The composite material 30 provideswhite light emission to a matrix addressable liquid crystal optical gate64 and a picture element (pixel) display is produced. FIG. 6 illustratesa monochrome liquid crystal display. For a full color liquid crystaldisplay, three color filters (e.g., blue, red, and green) may berequired for each pixel.

[0039]FIG. 7 illustrates a light emitting diode (LED) 70 having variouscolors. The power supply 52 excites the post-processed composite 30which emits white light. The white light emission is then transmittedthrough a color filter 72 (or color filters) which provides the colorfrom the LED.

[0040]FIG. 8 illustrates a pixel 80 of a full color electroluminescentdisplay composed of a plurality of sub-pixels 83 each having apost-processed composite 30. Transmission of the white light from one ofthe post-processed composites 30 through red, green or blue filters 84provides the color from an individual pixel 80. Power supply 52 iscoupled to a controller 82 which specifies which composite 30 isenergized and, therefore, specifies which color is emitted from thepixel 80. The controller 82 may be a matrix addressing circuit capableof distributing the energy from the power supply 52 to each sub-pixel83.

[0041] An advantage of the exemplary embodiments disclosed herein isthat the spectral makeup of the white light emission may be controlledby a simple and inexpensive irradiation process.

[0042] Another advantage of the exemplary embodiments is that theresulting processed composite will have shorter wavelength operationwhich is enabled by the blue-shifted quantum confined energy levels.

[0043] The foregoing is illustrative of the present invention and is notto be construed as limiting thereof. The invention is defined by thefollowing claims, with equivalents of the claims to be included therein.

1. A method comprising: directing an energy beam at a pre-processedcomposite material having a matrix containing a plurality ofnanocrystals and a plurality of traps to reduce the size of saidplurality of nanocrystals and the number of the plurality of traps toproduce a post-processed composite material.
 2. The method of claim 1,wherein said nanocrystals are from the group consisting of Group II-VI,Group III-V and Group IV semiconductor materials capable of emittingvisible light upon excitation.
 3. The method of claim 1, wherein saidnanocrystals comprises ZnSe.
 4. The method of claim 1, wherein saidplurality of traps are from the group consisting of impurities that arecapable of producing light emission upon excitation.
 5. The method ofclaim 1, wherein said traps are from the group consisting of Semolecules, Se vacancies, or zinc vacancies.
 6. The method of claim 1,wherein said matrix material is a transparent material that may containsaid nanocrystals and traps that emit in the visible light range whenfluoresced.
 7. The method of claim 1, wherein said matrix materialcomprises a glass material.
 8. The method of claim 1, wherein saidmatrix material comprises a potassium borosilicate material.
 9. A methodcomprising: directing an energy beam at a first state composite materialhaving a plurality of nanocrystals and a plurality of traps to reducethe size of the nanocrystals and the number of said plurality of trapsto produce a second state composite material capable of white lightemission when fluoresced.
 10. A method comprising: fluorescing apre-process composite material having a plurality of nanocrystals and aplurality of traps to obtain a light emission spectrum; performing ananalysis of said light emission spectrum; directing an energy beam atthe pre-process composite material to reduce the size of the pluralityof nanocrystals and to reduce the number of the plurality of traps toproduce a post-process composite material capable of white lightemission when fluoresced.
 11. A method of controlling the white lightemission of a composite material comprising: irradiating with an energybeam said composite material to reduce the size of a plurality ofnanocrystals positioned in said composite material and to reduce thenumber of a plurality of traps positioned in said composite material.12. A method of controlling the white light emission of a compositematerial comprising: laser irradiating said composite material to reducethe size of a plurality of blue light emitting nanocrystals and toreduce the number of red and green light emitting traps.
 13. A methodcomprising: 1) fluorescing a pre-process composite material having aplurality of nanocrystals and a plurality of traps to obtain a lightemission spectrum; 2) performing an analysis of said light emissionspectrum; 3) directing an energy beam at the pre-process compositematerial to reduce the size of the plurality of nanocrystals and toreduce the number of the plurality of traps to produce a post-processcomposite; and 4) repeating steps 1, 2 and 3 until a predetermined whitelight emission is obtained.
 14. A method of tailoring white lightemission from a composite having optical properties using ZnSenanocrystals comprising: fabricating said ZnSe nanocrystals;incorporating said ZnSe nanocrystals into a matrix to form a composite;and tuning the optical properties of said composite to a predeterminedapplication.
 15. The method of claim 14, wherein said optical propertiescomprise quantum confined bandedge emission from the ZnSe nanocrystals.16. The method of claim 14, wherein said tuning of said opticalproperties is conducted by irradiating said composite.
 17. The method ofclaim 14, wherein said tuning of said optical properties increases theefficiency of the spectral yield of said white light emission byoptimizing the number density of the ZnSe nanocrystals.
 18. The methodof claim 14, wherein said tuning step controls contribution of a bluespectral region of the white light emission from quantum confinedbandedge emission of the ZnSe nanocrystals.
 19. The method of claim 14,wherein said tuning step controls contribution of blue, red and greenportions of the white light emission by controlling the size of the ZnSenanocrystals and the number of traps in the composite.
 20. The method ofclaim 14, wherein said tuning of said optical properties comprises laserirradiating said composite to control amounts of red and green emissionfrom a plurality of traps and to control blue bandedge emission fromsaid ZnSe nanocrystals.
 21. The method of claim 14, wherein saidfabricating step further comprises incorporating said ZnSe in aninterface material located between matrix and said ZnSe nanocrystals;and wherein said interface material is a transparently visible material.22. The method of claim 21, wherein said interface material comprisesglass.
 23. The method of claim 22, wherein said glass material comprisesa potassium borosilicate.
 24. The method of claim 14, wherein saidfabricating step further comprises incorporating said ZnSe in aninterface material located between matrix and said ZnSe nanocrystals;and wherein said interface material comprises a material capable ofhaving nanocrystals embedded within.
 25. The method of claim 14, whereinsaid fabricating step further comprises incorporating said ZnSe in aninterface material located between said matrix and said ZnSenanocrystals; and wherein said interface material comprises trapscapable of emitting light in the red and green region of the spectrum.26. The method of claim 14, wherein said fabricating step furthercomprises incorporating said ZnSe in an interface material locatedbetween matrix and said ZnSe nanocrystals; and wherein said interfacematerial is a polymer.
 27. The method of claim 14, wherein said polymercomprises polystyrene.
 28. The method of claim 14, wherein saidfabricating step further comprises incorporating said ZnSe in aninterface material located between matrix and said ZnSe nanocrystals;and wherein said interface material is a sol-gel.